Recently, conductive adhesive, instead of solder, is widely used for assembling a semiconductor element or various electrical/electronic components, or for adhering them to a substrate.
For example, the following patent document 1 describes a conductive adhesive having a sufficient strength and conductivity, the adhesive comprising metal fillers containing copper, an epoxy compound, a novolak phenolic resin, a low molecular weight polyphenolic compound, and a curing agent, as indispensable components. Further, Patent Document 2 describes a conductive adhesive having a superior adhesion strength, the adhesive containing an epoxy resin and a phenolic resin which are liquid at a room temperature.
Further, an anisotropic conductive film having conductive particles dispersed in a resin may also be used for electrical connection between circuit substrates, or between an electronic component, such as an IC chip, and a circuit substrate. The anisotropic conductive film is arranged between electrodes of the electronic components opposing to each other, and heat and pressure are applied thereto, so that the electrodes can be adhered and electrically connected to have conductivity in the pressurization direction.
As an example of such an anisotropic conductive film, for example, Patent Document 3 discloses an anisotropic conductive film comprising a laminate of a first adhesive film layer and a second adhesive film layer, the first adhesive film layer containing a polymerized photopolymerizable resin, a thermosetting resin, a curing agent for the thermosetting resin, and conductive particles, and the second adhesive film layer containing a thermosetting resin, and a curing agent for the thermosetting resin. Patent Document 4 discloses an anisotropic conductive film comprising conductive particles dispersed in an insulative adhesive containing a silane coupling agent.
There has been a demand for mounting components on a wiring pattern prepared by applying photolithography on an aluminum- or copper-foil clad PET (polyethylene terephthalate) film, or prepared by screen printing or stencil printing using a silver paste. However, if a conventional epoxy conductive adhesive is used, the adhesive strength is high, but curing at a low temperature is difficult because such an epoxy conductive adhesive usually heated at a high temperature of 160 to 200° C. in order to obtain both the preservation stability and the curability. When the thermosetting conductive adhesive is cured at a low temperature of approximately 150° C., connection is possible, but sufficient connection reliability cannot be easily obtained because the resin has a low moisture resistance and heat resistance.
Therefore, it has been desired that a thermosetting conductive adhesive has not only a superior adhesive property and connection reliability, but also curability at a low temperature. For example, Patent Document 5 discloses an anisotropic conductive adhesive containing conductive particles dispersed in an adhesive resin composition which comprises a polyester resin having a glass transition temperature of 40° C. or less, blocked isocyanate (B), epoxy acrylate (C), and a polymerization initiator (D).
The epoxy acrylate used herein is synthesized using an epoxy resin as a raw material and reacting the epoxy resin with a (meth)acrylic acid. The epoxy resin, i.e., raw material, contains organic chlorine derived from epichlorohydrin, and the epoxy acrylate has a higher viscosity than the epoxy resin. Unlike the epoxy resin, the epoxy acrylate cannot be mixed with a liquid curing agent having a low viscosity. Accordingly, this may cause problems in mixing conductive particles, particularly in case of a conductive adhesive in which conductive particles such as silver are mixed at a high concentration. There are problems that if polyfunctional (meth)acrylic acid ester is mixed in order to lower the viscosity, the adhesive property is decreased, and if monofunctional (meth)acrylic acid ester is mixed therefor, both the heat resistance and the adhesive property are decreased.